Magnetic resonance imaging (MRI) is becoming increasingly well accepted for medical diagnostics because of its advantages over computerized axial tomography scanners which subject the patient undergoing examination to potentially harmful x-rays. However, MRI magnets, without special shielding, provide relatively high magnetic flux density outside the bore of the magnet also. Space in hospitals is at a premium and high strength magnetic fields can cause interference with other hospital diagnostic equipment as well as with control devices such as cardiac pacemakers and neuro stimulators. Thus, it is necessary to provide shielding for the magnet, particularly in view of a United States Food and Drug Administration ruling that there be an area of exclusion provided at the five gauss flux line.
There are several ways to provide shielding for an MRI magnet. One is active shielding in which additional coils are employed so as to provide magnetic fields in opposition to the field provided by the magnet. Such additional coils are placed outside the magnet coil (in the helium cryostat) to reduce the field strength in the patient volume and cancel the field outside of the magnet. Besides the expense of the additional coils, which can add up to thirty percent to the cost of a magnet system, siting a magnet system employing active shielding presents severe problems. The magnet system is large, heavy and must be located as a single unit. It cannot be assembled from components at the site. Field uniformity in the bore must be provided for, for example, by the addition of shimming both inside and outside the bore to achieve the required magnetic field homogeneity.
The nominal bore diameter for an actively shielded magnet system is 100 cm. Such a large bore requires a large magnet main coil to provide sufficient flux to achieve the required high flux density in the bore. The large bore magnet system with a large main coil, also with the active shielding coils and attendant frame structure, results in a magnet system which weighs in the range of 15,000-20,000 lbs. Not only the site but also the floor of the path taken to the site, for example, in a hospital must be able to bear this load. The doorways through which the magnet system passes must be sufficiently large to permit its passage or expensive enlargement of the doorways must be provided.
Passive shielding is another method of limiting the field outside of the magnet. One type of passive shielding involves forming a room about the magnet with the walls of the room containing ferromagnetic material to provide a high permeability return path for the magnetic lines of force. This method of shielding has shortcomings in that it requires custom engineering and design for each site location and thus becomes very expensive. The weight of the iron in the walls may require structural reinforcement.
A second type of passive shielding entails placing a shield of ferromagnetic material in close proximity to the vacuum vessel of the magnet. Such a shield, of iron or low carbon steel, provides the high permeability return path for the lines of force to greatly limit the intensity of the field outside the magnet. Unfortunately, the presence of so much iron near the bore of the magnet greatly degrades the degree of homogeneity of the field inside the magnet bore. For example, a magnet providing a homogeneity of 5 parts per million (PPM) without the shield may provide 5000 ppm in the presence of the shield. Such a degree of nonuniformity is not suitable for MRI. In order to improve the degree of homogeneity inside the magnet bore, various correction or error coils (shim coils) are wound. It will be appreciated that the winding of such shim coils with respect to an existing magnet is time consuming and expensive. The presence of the shim coils occupies precious space in the bore and adds weight to the magnet system. For additional information regarding the structure and operation of prior art MRI magnets having shields and shim coils, reference may be made to U.S. Pat. Nos. 4,490,675; 4,590,452; 4,612,505 and 4,646,045.